1. Field of the Invention
The present invention relates to a surface inspection apparatus for detecting defect on a surface of a magnetic disk substrate. More particularly, the present invention relates to a surface inspection method and apparatus for measuring the shape of a defect through phase detection.
2. Description of the Related Art
A disk substrate with evaporated magnetic substance is used as a magnetic recording medium for a hard disk drive. Data is magnetically written and read by magnetizing the disk substrate by use of a magnetic head. In recent years, with an increase in the recording density of hard disk drive, the space (hereinafter referred to as flying height) between a recording head (hereinafter referred to as head) and the disk substrate is becoming much narrower, i.e., from several tens of nanometers to several nanometers. Therefore, if minute concavo-convex defect exists on the disk substrate, the head comes in contact with the disk substrate which may cause failure of the hard disk drive.
Therefore, it is important to detect the above-mentioned defect on a substrate before evaporating the magnetic substance to prevent a defective product from being transferred to subsequent processes. This minute concavo-convex defect is possibly a crystal defect buried in the disk substrate material, a residual abrasive particle or a fine flaw (scratch or the like) produced in polishing process performed to improve the flatness of the disk substrate, or a foreign material which adheres to the surface in washing or drying process. Foreign substances adhering to the surface can be eliminated and prevented through re-washing, purification of atmosphere, or other appropriate measures. However, since crystal defect, a scratch, etc. are unrepairable, a product with such defect will be handled as a defective one. Therefore, in order to ensure high yield and high reliability of hard disk drive, early elimination of disk substrates having such defect is essential. Furthermore, since the above-mentioned defect may occur because of a certain reason even after magnetic substance has been evaporated, it is possible to improve the reliability of hard disk drive by similarly detecting defect and eliminating defective products.
Conventionally, measurement of surface roughness based on an AFM (atomic force microscope) is commonly used as a method for measuring minute concavo-convex defect. In AFM-based measurement, the throughput is remarkably slow and therefore it is not easy to perform entire surface inspection. Furthermore, this method has some drawbacks: for example, a measurement probe is a consumable; measurement reproducibility fluctuates because of a worn probe, etc.
Therefore, there is a method adapting interference of light. As shown in JP-A-2000-121318, there is a method for measuring the height of defect from variation of phase by performing the steps of branching a laser beam, modulating branched light beams with different frequencies, irradiating reference and measurement surfaces with the laser beams, and allowing respective reflected light beams to interfere on a light-sensitive element. In accordance with above-mentioned JP-A-2000-121318, a laser beam with a lighting wavelength of 532 nm is used, an interference signal with a frequency of 10 MHz is outputted, and the change of an optical path length of the measurement surface is converted into a phase, thus enabling measurement of the height of defective from the phase difference.
By the way, as a laser beam used for an interference optical system described in above-mentioned JP-A-2000-121318, a highly coherent narrow-band laser having a narrow spectrum width (such as gas laser and diode-pumped solid state laser) is used. The coherence length is several tens of millimeters to several meters, which causes no problem on interference in terms of the optical path length of the reference and measurement surfaces. FIG. 9 shows a spectrum distribution of a narrow-band laser with a wavelength of 532 nm which is a diode-pumped solid state laser. Thus, a narrow-band laser has a spectrum of 1 nm or less, generally giving a coherence length ranging from several tens of millimeters to several meters.
However, since the shape of target defect is becoming more and more minute as mentioned above, the improvement in the detection sensitivity is required. In the case of the interference-phase measurement scheme, since the period of an interference signal is a half of the laser wavelength, the detection sensitivity is determined by the laser wavelength. Therefore, to improve the detection sensitivity, it is necessary to shorten the wavelength of a laser light source. To shorten the wavelength of the laser light source, highly advanced adjustment is required using a crystal according to each wavelength to obtain a plurality of high order harmonics. As the wavelength is shortened, therefore, the structure of the laser light source becomes more complicated resulting in remarkably high price. For this reason, there arises a subject of increased price of a surface inspection apparatus.
However, technology for shortening the wavelength has progressed even in the case of diode laser making it easier to obtain a low-price high-power light source. However, diode laser has a larger spectrum width than the above-mentioned diode-pumped solid state laser. In particular, diode laser with a shortened wavelength has a subject of a larger spectrum width than diode laser with a long wavelength. Thus, since the coherence length becomes remarkably shorter with increasing spectrum width, diode laser is not suitable as a light source for the above-mentioned interference optical system.
On the other hand, there is a method for narrowing a band of diode laser having a large spectrum width using a diffraction grating. With this method, a specific wavelength is taken out from the diffraction grating and therefore the output is remarkably reduced and at the same time the arrangement of the diffraction grating is delicate. There arises a subject of aging as well as a subject that laser beam cannot be taken out because of vibratory effect on the diode laser light source. Furthermore, since the configuration of the diode laser light source may become complicated, there arises a subject of high price.